Technical Field
This invention relates to a measurement system and a method for measuring parameters of a body tissue, such as the determination of blood volume and blood flow in the brain and the muscles or the measurement of the intracranial pressure. In particular the invention relates to a non-invasive measurement of cerebral and muscular parameters.
Related Art
The content of the cranium is composed of the non-compressible components brain, cerebral blood volume (CBV) and cerebrospinal fluid (liquor cerebrospinalis; CSF). After exhaustion of the intracranial compensation mechanisms, such as e.g. reduction of the cerebrospinal fluid production, decrease of the CBV, processes requiring space lead to an increase in intracranial pressure. These circumstances follow e.g. from the Monroe-Kellie doctrine. With increase of the intracranial pressure (ICP) over 50 mmHg, the cerebral perfusion pressure (CPP) can no longer be maintained, and additional damage to brain areas results through a global brain ischemia, such as e.g. is known from “Analysis of Intracranial Pressure: Past, Present and Future”, A. Di Ieva et al; The Neuroscientist; 6 Feb. 2013. The CPP is thereby identified as the difference between the mean arterial pressure and the ICP. Understood by intracranial pressure, ICP, is by definition the supratentorial cerebrospinal fluid pressure, i.e. the pressure in the lateral ventricles and in the subarachnoid space over the cerebral convexity. According to the conventional view, ICP rises are due to a swelling of the brain, e.g. in the case of a defective blood-cerebral barrier, or a cytotoxic cerebral edema, such as an intracellular water accumulation, an additional space requirement, e.g. through tumor, hemorrhage, etc., or a cerebrospinal fluid, circulatory or respectively absorption disorder.
Known from the state of the art are various methods of measuring the intracranial pressure in the skull and of measuring parameters derived indirectly therefrom. For this purpose invasive methods are conventionally used in which a measuring probe is introduced through the skull into the brain tissue. Invasive methods have however various drawbacks, such as undesired bleeding, complicated execution of the method, locally limited measuring regions, etc.
Furthermore non-invasive measuring methods are used which are based e.g. on ultrasound and Doppler effect and use a correlation between an increased intracranial pressure and the pulsatility. However there exists a great variation in the pulsatility index, even in healthy people. In another method principles of fluid mechanics and neurophysiology are combined with results from magnetic resonance images, as described for example in “Noninvasive intracranial compliance and pressure based on dynamic magnetic resonance imaging of blood flow and cerebrospinal flow,” Patricia B. Raksin et al., Neurosurg. Focus, Volume 14. Known from EP 0933061 B1 is a measuring configuration in which a proportionality between the intracranial pressure and a pressure existing in the external auditory canal is used for determining the intracranial pressure. Furthermore shown in US 2012/0136240 A1 is a system in which a controlled constriction of the jugular vein leads to a change in the blood outflow and the pressure of the blood outflow is measured, whereby the intracranial pressure is determined from the relationship between constriction and blood flow. Used thereby are e.g. near-infrared (NIR) sensors for determining the blood outflow. Furthermore absorption measuring methods in the near-infrared range are known for monitoring constituents of an organ, e.g. of the brain, in particular for monitoring oxygenated and deoxygenated blood, or for determining a concentration of oxygenated and deoxygenated blood e.g. in the brain, such as shown in U.S. Pat. No. 6,195,574 or 5,706,821.
A comparable situation is to be found with injuries and traumas of muscles that are tightly disposed in fascia, such as e.g. a lower leg or lower arm muscle. After a trauma, owing to compartmentally increased pressure inside a fascia, there exists a high risk that the muscle dies since blood circulation through the muscle is diminished by the pressure increase. The patients suffer a loss of muscles or even a life-threatening rhabdomyolysis syndrome. It is thus desirable to determine and to be able to monitor the pressure parameters in the muscles.
With the known methods, complicated algorithms and complex measurement configurations are required for determining pressure parameters in the body tissue. Moreover they are focused on specific parameters and are limited to few measurement sites, such as e.g. the ear. For measuring various parameters of a body tissue therefore different measuring systems must be used simultaneously or in succession. In addition, also the known non-invasive methods can be unpleasant and risky for the patient.